Forward declaration of nested types/classes in C++

Calmarius_58805 0
Viewed 64998

I recently got stuck in a situation like this:

class A
{
public:
    typedef struct/class {…} B;
…
    C::D *someField;
}

class C
{
public:
    typedef struct/class {…} D;
…
    A::B *someField;
}

Usually you can declare a class name:

class A;

But you can't forward declare a nested type, the following causes compilation error.

class C::D;

Any ideas?

forward-declaration class nested c++
7 Answers

You can't do it, it's a hole in the C++ language. You'll have to un-nest at least one of the nested classes.

Adam_Rosenfield_95300

If you really want to avoid #including the nasty header file in your header file, you could do this:

hpp file: 

class MyClass
{
public:
    template<typename ThrowAway>
    void doesStuff();
};

cpp file

#include "MyClass.hpp"
#include "Annoying-3rd-party.hpp"

template<> void MyClass::doesStuff<This::Is::An::Embedded::Type>()
{
    // ...
}

But then:

  1. you will have to specify the embedded type at call time (especially if your function does not take any parameters of the embedded type)
  2. your function can not be virtual (because it is a template)

So, yeah, tradeoffs...

user_13363400

I would not call this an answer, but nonetheless an interesting find: If you repeat the declaration of your struct in a namespace called C, everything is fine (in gcc at least). When the class definition of C is found, it seems to silently overwrite the namspace C.

namespace C {
    typedef struct {} D;
}

class A
{
public:
 typedef struct/class {...} B;
...
C::D *someField;
}

class C
{
public:
   typedef struct/class {...} D;
...
   A::B *someField;
}
user_136770

If you have access to change the source code of classes C and D, then you can take out class D separately, and enter a synonym for it in class C:

class CD {

};

class C {
public:

    using D = CD;

};

class CD;
user_45966130

This can be done by forward declare the outer class as a namespace.

Sample: We have to use a nested class others::A::Nested in others_a.h, which is out of our control.

others_a.h

namespace others {
struct A {
    struct Nested {
        Nested(int i) :i(i) {}
        int i{};
        void print() const { std::cout << i << std::endl; }
    };
};
}

my_class.h

#ifndef MY_CLASS_CPP
// A is actually a class
namespace others { namespace A { class Nested; } }
#endif

class MyClass {
public:
    MyClass(int i);
    ~MyClass();
    void print() const;
private:
    std::unique_ptr<others::A::Nested> _aNested;
};

my_class.cpp

#include "others_a.h"
#define MY_CLASS_CPP // Must before include my_class.h
#include "my_class.h"

MyClass::MyClass(int i) :
    _aNested(std::make_unique<others::A::Nested>(i)) {}
MyClass::~MyClass() {}
void MyClass::print() const {
    _aNested->print();
}
user_9364370
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